air pollution
What is air pollution?
What are some common sources of air pollution?
How does air pollution affect the environment?
What health problems can be caused by air pollution?
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air pollution, release into the atmosphere of various gases, finely divided solids, or finely dispersed liquid aerosols at rates that exceed the natural capacity of the environment to dissipate and dilute or absorb them. Air is considered to be polluted when it contains certain substances in concentrations high enough and for durations long enough to cause harm or undesirable effects. These include adverse effects on human health, property, and atmospheric visibility. The atmosphere is susceptible to pollution from natural sources as well as from human activities. Some natural phenomena, such as volcanic eruptions and forest fires, may have not only local and regional effects but also long-lasting global ones. Nevertheless, only pollution caused by human activities, such as industry and transportation, is subject to mitigation and control.
Air pollution is one of the world’s leading environmental health risks. According to the World Health Organization (WHO), 99 percent of the global population is exposed to air pollution levels that exceed recommended health guidelines. Exposure to polluted air has been linked to a wide range of health problems, including respiratory and cardiovascular disease, asthma, stroke, and lung cancer, and is estimated to contribute to roughly seven million premature deaths each year. The health burden is greatest in densely populated urban and industrial regions, particularly in low- and middle-income countries, though air pollution affects communities worldwide.
Local air quality and air movement
Local air quality typically varies over time because of the effect of weather patterns. For example, air pollutants are diluted and dispersed in a horizontal direction by prevailing winds, and they are dispersed in a vertical direction by atmospheric instability. Unstable atmospheric conditions occur when air masses move naturally in a vertical direction, thereby mixing and dispersing pollutants. When there is little or no vertical movement of air (stable conditions), pollutants can accumulate near the ground and cause temporary but acute episodes of air pollution. With regard to air quality, unstable atmospheric conditions are preferable to stable conditions.
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The degree of atmospheric instability depends on the temperature gradient (i.e., the rate at which air temperature changes with altitude). In the troposphere (the lowest layer of the atmosphere, where most weather occurs), air temperatures normally decrease as altitude increases; the faster the rate of decrease, the more unstable the atmosphere. Under certain conditions, however, a temporary “temperature inversion” may occur, during which time the air temperature increases with increasing altitude, and the atmosphere is very stable. Temperature inversions prevent the upward mixing and dispersion of pollutants and are the major cause of air pollution episodes.
Certain geographic conditions exacerbate the effect of inversions. For example, Los Angeles, situated on a plain on the Pacific coast of California and surrounded by mountains that block horizontal air motion, is particularly susceptible to the stagnation effects of inversions—hence the infamous Los Angeles smog. On the opposite coast of North America, another metropolis, New York City, produces greater quantities of pollutants than does Los Angeles but has mostly been spared major air pollution disasters—only because of favorable climatic and geographic circumstances. During the mid-20th century, governmental efforts to reduce air pollution increased substantially after several major inversions, such as the Great Smog of London, a five-day air pollution episode in London in 1952 that was directly blamed for more than 4,000 deaths.
Air quality monitoring
Around the world, government agencies, research institutions, and international organizations operate networks of monitoring stations that continuously measure levels of air pollutants. Data from these stations are used to evaluate compliance with air quality standards, identify pollution trends, and inform public health responses. Ground-based monitoring stations provide direct measurements of pollutant concentrations at specific locations, while portable sensors can be used to assess local conditions and short-term pollution events. In addition, aircraft and satellites equipped with remote-sensing instruments provide regional and global observations of atmospheric pollutants, allowing scientists to track pollution transport, wildfire smoke, dust storms, and long-term environmental change. Monitoring data play an important role in air quality forecasting, pollution control policy, and scientific research.
Many countries communicate daily air quality conditions through the Air Quality Index (AQI), a color-coded index that indicates whether the relative air quality and air pollution levels in a given area are healthy or unhealthy. The AQI and similar systems typically combine measurements of several major pollutants into a single scale that indicates the potential health risks associated with outdoor air exposure. Higher index values correspond to poorer air quality and greater health concerns, particularly for sensitive populations such as children, older adults, and individuals with respiratory or cardiovascular diseases.
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Major air pollutants
Clean, dry air consists primarily of nitrogen and oxygen—78 percent and 21 percent, respectively, by volume. The remaining 1 percent is a mixture of other gases, mostly argon (0.9 percent), along with trace amounts of carbon dioxide, methane, hydrogen, helium, and more. Water vapor is also a normal, though quite variable, component of the atmosphere, normally ranging from 0.01 to 4 percent by volume; under very humid conditions the moisture content of air may be as high as 5 percent.
Air pollutants are often classified as either primary or secondary pollutants. Primary pollutants are released directly into the atmosphere from identifiable sources such as automobiles, power plants, factories, and wildfires. Examples include sulfur dioxide, nitrogen oxides, carbon monoxide, lead particulates, and many volatile organic compounds. Secondary pollutants form in the atmosphere through chemical reactions involving primary pollutants and naturally occurring atmospheric components such as oxygen and water vapor. Ozone in photochemical smog and sulfuric and nitric acids in acid rain are important examples of secondary pollutants. Distinguishing between primary and secondary pollutants is important because secondary pollutants may form far from the original emission source and can be influenced strongly by sunlight, temperature, and weather conditions.
Criteria pollutants
There are six major air pollutants that have been designated by the U.S. Environmental Protection Agency (EPA) as “criteria” pollutants—criteria meaning that the concentrations of these pollutants in the atmosphere are useful as indicators of overall air quality. The sources, acceptable concentrations, and effects of the criteria pollutants are summarized in the table.
| pollutant | common sources | maximum acceptable concentration in the atmosphere | environmental risks | human health risks |
|---|---|---|---|---|
| Source: U.S. Environmental Protection Agency | ||||
| carbon monoxide (CO) | automobile emissions, fires, industrial processes | 35 ppm (1-hour period); 9 ppm (8-hour period) | contributes to smog formation | exacerbates symptoms of heart disease, such as chest pain; may cause vision problems and reduce physical and mental capabilities in healthy people |
| nitrogen oxides (NO and NO2) | automobile emissions, electricity generation, industrial processes | 0.053 ppm (1-year period) | damage to foliage; contributes to smog formation | inflammation and irritation of breathing passages |
| sulfur dioxide (SO2) | electricity generation, fossil-fuel combustion, industrial processes, automobile emissions | 0.03 ppm (1-year period); 0.14 ppm (24-hour period) | major cause of haze; contributes to acid rain formation, which subsequently damages foliage, buildings, and monuments; reacts to form particulate matter | breathing difficulties, particularly for people with asthma and heart disease |
| ozone (O3) | nitrogen oxides (NOx) and volatile organic compounds (VOCs) from industrial and automobile emissions, gasoline vapours, chemical solvents, and electrical utilities | 0.075 ppm (8-hour period) | interferes with the ability of certain plants to respire, leading to increased susceptibility to other environmental stressors (e.g., disease, harsh weather) | reduced lung function; irritation and inflammation of breathing passages |
| particulate matter | sources of primary particles include fires, smokestacks, construction sites, and unpaved roads; sources of secondary particles include reactions between gaseous chemicals emitted by power plants and automobiles | 150 μg/m3 (24-hour period for particles <10 μm); 35 μg/m3 (24-hour period for particles <2.5 μm) | contributes to formation of haze as well as acid rain, which changes the pH balance of waterways and damages foliage, buildings, and monuments | irritation of breathing passages, aggravation of asthma, irregular heartbeat |
| lead (Pb) | metal processing, waste incineration, fossil-fuel combustion | 0.15 μg/m3 (rolling three-month average); 1.5 μg/m3 (quarterly average) | loss of biodiversity, decreased reproduction, neurological problems in vertebrates | adverse effects upon multiple bodily systems; may contribute to learning disabilities when young children are exposed; cardiovascular effects in adults |
The gaseous criteria air pollutants of primary concern in urban settings include sulfur dioxide, nitrogen dioxide, and carbon monoxide; these are emitted directly into the air from fossil fuels such as fuel oil, gasoline, and natural gas that are burned in power plants, automobiles, and other combustion sources. Ozone (a key component of smog) is also a gaseous pollutant; it forms in the atmosphere via complex chemical reactions occurring between nitrogen dioxide and various volatile organic compounds (e.g., gasoline vapors).
Airborne suspensions of extremely small solid or liquid particles called “particulates” (e.g., soot, dust, smoke, fumes, mists), especially those less than 10 micrometers (μm; millionths of a meter) in size, are significant air pollutants because of their very harmful effects on human health. They are emitted by various industrial processes, coal- or oil-burning power plants, residential heating systems, and automobiles. Lead fumes (airborne particulates less than 0.5 μm in size) are particularly toxic and were an important pollutant of many gasoline and diesel fuels before being globally eliminated. (Algeria was the last country to stop using leaded diesel in 2021.)
Except for lead, criteria pollutants are emitted in industrialized countries at very high rates, typically measured in millions of tons per year. All except ozone are discharged directly into the atmosphere from a wide variety of sources. They are regulated primarily by establishing ambient air quality standards, which are maximum acceptable concentrations of each criteria pollutant in the atmosphere, regardless of its origin. The six criteria pollutants are described in turn below.